To understand the complexity of signal transduction pathways involved in cell growth and differentiation, it is essential to be able to track protein expression, localization, and conformational changes. A first step in tracking these parameters in living cells is attaching a biophysical probe such as a fluorophore to the biomolecule or biomolecules of interest. Traditional chemical labeling methods, unfortunately, are nonspecific and often result in complicated data with high background noise. For overcoming the in vivo specificity challenge in labeling, it is proposed that the exquisite substrate specificity of the E. coli's biotin transferase enzyme be harnessed to catalyze covalent labeling. By using phage display, this enzyme's substrate specificity and catalytic activity may be evolved to convert it into a fluorophore transferase, which in turn, would expand the scope of compatible probe structures for site-specific labeling of recombinant proteins.